Environmental and economic concerns associated with depleting oil resources have triggered a growing interest in the chemical conversion of carbon dioxide (CO2), so as to enable its use as a renewable carbon source. CO2 is, despite its low reactivity, a highly attractive carbon feedstock, as it is inexpensive, virtually non-toxic, abundantly available in high purity and non-hazardous. Therefore, CO2 could be a promising substitute for substances such as carbon monoxide, phosgene or other petrochemical feedstocks in many processes. One of the developing applications of CO2 is the copolymerization with epoxides to yield aliphatic polycarbonates. The development of effective catalysts to make such a process profitable is the subject of continuous research.
In WO2009/130470, the contents of which are incorporated herein by reference in their entirety, the copolymerisation of an epoxide with CO2 using a catalyst of a class represented by formula (I) was described:

WO2013/034750, the contents of which are incorporated herein by reference in their entirety, discloses the copolymerisation of an epoxide with CO2 in the presence of a chain transfer agent using a catalyst of a class represented by formula (I):

Various compounds according to formula (I) above were tested for their ability to catalyse the reaction between different epoxides and carbon dioxide.
In each of these tested catalysts, both occurrences of R3 were the same and all occurrence of R4 were the same (referred to hereinafter as symmetric catalysts).
Among the epoxides employed in the copolymerization reactions of the prior art, cyclohexene oxide (CHO) received special interest, as the product, poly(cyclohexene carbonate) (PCHC) shows a high glass transition temperature and reasonable tensile strength. Ethylene oxide, propylene oxide and butylene oxide have also received interest as they produce polymers (polyalkylene carbonates, such as PPC) with elastomeric properties which are useful in many applications e.g. films.
The inventors have now surprisingly found that the asymmetric catalysts referred to herein represent a novel and inventive means of catalysing the polymerisation of carbon dioxide with various monomers to produce useful polymer products with good activity and selectivity.